1. Technical Field
This disclosure relates to voltage regulators, including error amplifiers used in them.
2. Description of Related Art
A voltage regulator may convert an input voltage to a regulated output voltage.
The voltage regulator may include an error amplifier. The error amplifier may compare a feedback signal, which may be proportional to the output voltage of the voltage regulator, with a reference voltage. The results of the comparison may be used to regulate the output voltage.
FIG. 1 is an example of a prior art voltage regulator which provides a positive output voltage based on a positive input voltage and which uses a combined error amplifier and driver circuit on a single integrated circuit. The LT1946 made by Linear Technology Corporation is an example of such an integrated circuit.
A driver circuit within the integrated circuit may deliver an output switching signal (SW) which may be used in the voltage regulator to effectuate output voltage regulation. The switching signal SW may be based in part on a comparison of the feedback signal FB with an internal reference voltage VREF by an error amplifier within the integrated circuit. The feedback signal FB may be generated by a resistor divider network R1 and R2 between the output voltage VOUT and ground.
FIG. 2 is an example of a prior art comparison circuit which may be used in the error amplifier illustrated in FIG. 1. As illustrated in FIG. 2, the reference voltage VREF may be generated internal to the integrated circuit. Differences between the reference voltage VREF and the feedback signal FB may be measured by amplifier A1 and may tell the error amplifier whether the output voltage VOUT is too high, too low, or just right, as indicated by the error output VC of the amplifier A1. The driver circuit within the integrated circuit in FIG. 1 may respond to this information by adjusting the timing of the switching signal SW, causing the output voltage VOUT to converge to the correct amount.
The configuration illustrated in FIG. 2 may be useful when the regulated output voltage VOUT is positive with respect to the ground GND of the combined error amplifier and driver circuit. However, when the output voltage VOUT is negative with respect to the ground GND of the combined error amplifier and driver circuit, this configuration may be problematic and a different configuration may be needed.
FIG. 3 is an example of a prior art voltage regulator which provides a negative output voltage based on a positive input voltage and which uses a combined error amplifier and driver circuit on a single integrated circuit. The LT1931 manufactured by Linear Technology Corporation is an example of such an integrated circuit.
FIG. 4 is an example of a prior art comparison circuit which may be used in the combined error amplifier and driver circuit illustrated in FIG. 3. As illustrated in FIG. 4, a different configuration for the comparison circuit may be needed because the output voltage is negative with respect to ground.
FIG. 5A is an example of a prior art comparison circuit which may be used in an error amplifier in a voltage regulator which may provide either a positive or negative output based on a positive input. FIG. 5A illustrates a three pin sensing configuration. FIG. 5B illustrates the circuit configured to sense positive output voltage. FIG. 5C illustrates the circuit configured to sense negative output voltage. Thus, the same comparison circuit in FIG. 5A may be configured to sense both positive and negative output voltages, while the comparison circuits illustrated in FIGS. 2 and 4 may only be able to sense output voltages having one of these polarities.
This increase in flexibility, however, may come at the expense of requiring two additional signal pins. When the error amplifier is contained on a single integrated circuit, this may add to the number of pins which must be dedicated to receiving the feedback signal.
FIG. 6A is another example of a prior art comparison circuit which may be used in an error amplifier in a voltage regulator which may provide either a positive or negative output based on a positive input. This configuration is described in more detail in U.S. Pat. No. 7,579,816, the entire contents of which is incorporated herein by reference. As described in more detail in this patent, FIG. 6A illustrates a two-pin sensing configuration. FIG. 6B illustrates this circuit configured to sense positive output voltages. FIG. 6C illustrates this circuit configured to sense negative output voltages. The configuration illustrated in FIGS. 6A-6C may provide similar flexibility to the configuration illustrated in FIGS. 5A-5C, but may require using only two signal pins, instead of three. U.S. Pat. No. 7,579,816 also describes a further configuration which also senses both positive and negative output voltages in a voltage regulator, but which requires only a single feedback pin.
All of the configurations which have been described, however, sense output voltage VOUT with respect to the ground pin (GND) on the combined error amplifier and driver circuit (when contained within an integrated circuit). However, there may be a need to generate output voltages which are regulated with respect to the power supply input pin VIN of the integrated circuit, rather than with respect to the ground (GND) of the integrated circuit.
FIG. 7 is an example of a prior art voltage regulator which provides a negative output voltage based on a negative input voltage and which uses a combined error amplifier and driver circuit on a single integrated circuit. FIG. 7 illustrates how an integrated circuit of the type having a sensing configuration similar to that illustrated in FIG. 2 may be adapted for regulating an output voltage which is negative with respect to the integrated circuit's input voltage VIN pin.
To facilitate this functionality, additional components, such as Q1, may be required to perform a level shifting function. In addition to costs, these additional components may introduce inaccuracies in the output voltage VOUT. These inaccuracies may be caused, for example, by manufacturing and temperature variations in the emitter-to-base voltage drop of Q1 and/or variations in the current gain of Q1.
FIG. 8 is an example of a prior art voltage regulator which provides a positive output voltage based on a negative input voltage and which uses a combined error amplifier and driver circuit on a single integrated circuit. FIG. 8 illustrates an adaptation of the FIG. 2 sensing scheme which regulates an output voltage VOUT which is positive with respect to the integrated circuit's VIN pin. This adaptation may suffer from the same inaccuracies discussed above in connection with FIG. 7.